A conventional flexographic printing station has a printing cylinder (also called a die cylinder) and an impression cylinder between which sheets, for example corrugated paper board blanks, are sequentially advanced for flexographic printing thereon. A printing die is mounted on the printing cylinder, for example by vacuum as disclosed in U.S. Pat. No. 4,744,297. Ink is applied to the printing die by an ink applicator roll (often referred to as an anilox roll) which has an engraved surface providing ink cells for holding ink to be transferred to the printing die. The ink may be uniformly squeegeed into the cells of the anilox roll by a rubber (or plastic) covered wipe roll rotating in tangential contact with the anilox roll. The ink is supplied to the top in-going nip of these rolls from an ink supply pipe so that a bead or reservoir of ink is formed in the nip. Excess ink runs off the ends of the rolls and falls into an ink pan below from which it flows through a drain back to the ink supply system.
Both the anilox and wipe rolls are conventionally mounted on inner frames pivotally movable with respect to a main frame of the printing station. Air cylinders can be used to pivot the anilox roll into and out of engagement with the printing cylinder. When blanks are not being advanced, the inner frames are pivoted away from the printing cylinder to stop the transfer of ink to the printing die, so avoiding build up Of ink on the die while the printing cylinder continues to rotate or is slowing to a halt, as well understood by those skilled in the art. When pivoted away from the printing cylinder, a main clutch in the drive train from the printing cylinder to the anilox roll is disengaged so that the printing cylinder does not drive the anilox roll.
The wipe roll can be driven by a wipe roll drive motor at a constant speed slower than the speed of the anilox roll when the anilox roll is rotated at maximum machine speed, for example the speed ratio between the anilox roll and the wipe roll may be approximately 8:1. This provides a wiping action between the surfaces of the rolls to squeegee the ink into the ink cells of the anilox roll. If a main machine containing the printing station is run at less than maximum speed, this ratio between the rolls decreases proportionately. When the machine is stopped, the wipe roll may continue to run at constant speed with friction between the roll surfaces also causing the anilox roll to turn at wipe rolls speed, the main drive clutch having been disengaged. Running the rolls at idle speed keeps water-based flexographic ink from drying on the roll surfaces.
The wipe roll can also be pivotally mounted and held against the anilox roll during operation. When the anilox roll is pivoted away from the printing cylinder, contact can be maintained between these rolls so that the bead or reservoir of ink does not fall through the nip. However, when the system is to be cleaned prior to changing ink colors, air cylinders are actuated to move the wipe roll away from the anilox roll to dump the ink into the ink pan for draining back to the ink supply system.
Anilox rolls with different surface screens are available, e.g. surfaces formed with small pyramids, or quadrangles, or hexagon shapes, or having channels therein etc. These differently engraved anilox rolls can provide different printing qualities. When installing a new printing station, the anilox roll may be chosen which best suits the majority of the printing intended to be performed at that printing station.